The present invention relates to devices for providing a modulated fluid pressure control signal in response to an electrical control signal input upon connection of the device to a source of pressurized fluid. In particular, the invention relates to devices employed for providing modulation of hydraulic fluid pressure for controlling the shift functions of an automatic transmission for vehicular drive trains.
In designing automotive power plants for meeting present day fuel economy and emission standards, it has been found necessary to integrate the control of engine operating parameters with the shifting of the transmission in order to effect minimum fuel consumption and the lowest level of engine exhaust emissions for given vehicle operating modes. In order to provide such integrated control, it has been found desireable to utilize solid state electronic controllers to process information from various engine sensors to derive an electrical control signal for controlling engine operations. The consequent availability of an intelligent or sophisticated electrical control signal for drive-train control, has thus made it desireable to provide some means or way of electrically controlling the shifting of the transmission, utilizing the output of the electronic engine controller.
One convenient way of controlling the shifting of a vehicle automatic transmission employs the technique of changing or varying the pressure of the hydraulic fluid in the shift control fluid circuit of the transmission.
In designing control devices for varying the fluid pressure in a transmission shift control circuit, it is known to provide a transducer in which a solenoid coil driver is operative to provide, in response to a varying electrical control signal, linear movement of an armature for moving a hydraulic spool valve. The movement of the spool valve controls flow to and from a control signal pressure chamber for providing an output fluid pressure control signal to the transmission shift control circuit. The pressure in the control signal chamber is applied to the end of the spool valve for counteraction against a spring biasing the armature and spool in a given direction.
Additionally, the known spool type valves require precision machining and a close clearance fit about the spool which yields slow response and suscepibility to binding from contaminants.
In the known transducer for providing a fluid pressure output signal responsive to a varying input electrical control signal, the force of the fluid pressure acting against the end of the spool valve requires a substantial counter-balancing force from the armature bias spring in order to maintain the valve spool in a balanced or equilibrium state. The resulting stiffness or relatively high spring rate of the armature bias spring necessitates a substantial magnetomotive force output from the electromagnetic coil. The force requirement has necessitated an electrical input control signal having a power level requirement higher than the capability of the electronic programmer provided for the engine control system.
Thus, it has been desired to find a way or means of providing a fluid pressure control signal transducer capable of operating on a low power input variable electrical control signal. In particular, it has been desired to find a transducer capable of operating on a low power level electrical control signal for providing a modulated fluid pressure signal for the hydraulic circuit in an automotive automatic transmission. This device may also be employed for controlling fluid pressure in hydraulically actuated clutches.